The scaphoid is a bone of the wrist aligned with the thumb which is located in the proximal carpal row of bones, and the second or distal carpal row of bones contains the trapezium which is attached to the metacarpal bone of the thumb. In a case of fracture or severe arthritic pain at the base of the thumb, there has been some tendency to remove the arthritic articular surfaces of the joint between the trapezium and this metacarpus (which is referred to as the CMC joint) or to remove the entire trapezium; however, when this occurs, stability is lost. Burton and Pellegrini have described a tendon interpositional arthroplasty technique for removing the trapezium and preventing the base of the metacarpus from rubbing on the scaphoid, see Burton et al. “Surgical Management of Basal Joint Arthritis of the Thumb. Part II. Ligament Reconstruction with Tendon Interposition Arthroplasty,” J. Hand Surg 11A:324-32 (1986), however, the results are felt to be less than satisfactory. U.S. Pat. No. 6,436,146 (Aug. 20, 2002) shows a wide variety of prosthetic implants designed for use in orthopedic joints, particularly joints of the hand, many of which have convex opposite surfaces; these suggested implants have a variety of different configurations in transverse sectional view and may be circular or oval in plan view. In FIG. 18B, there are shown 2 implants for CMC joints, which have the shapes shown in either FIG. 2A or 2C or FIGS. 10A and 10B. However, in the region of the CMC joint, it is felt that the resultant overall stability of such an implant shape is insufficient for patient satisfaction. Better solutions to this problem have thus been sought.